The present invention relates to temperature control systems for integrated circuit chips (IC-chips). More particularly, the present invention relates to temperature control systems which circulate a fluid thru heat exchangers that are coupled to the IC-chips such that the temperature of the IC-chips stays within a few degrees of a selectable set point while the IC-chips undergo large step increases and large step decreases in power dissipation as they are tested.
After an IC-chip is initially fabricated, it must be tested in order to determine if all of the circuitry which is in the IC-chip operates properly. This testing is often done via a xe2x80x9cburn-inxe2x80x9d test wherein the IC-chip is kept above its normal operating temperature while it is sent a series of test signals. Such a burn-in test is performed because it greatly shortens the time period during which certain types of failures will occur within the IC-chip, if those failures are going to occur at all.
In the prior art, the burn-in test usually is performed by the steps of 1) placing multiple IC-chips in sockets on several printed circuit boards, 2) moving the printed circuit boards that are holding the IC-chips into an enclosed tester which has a heater, and 3), blowing hot air from the heater with fans such that the hot air flows across the IC-chips while they are sent the test signals. Such an enclosed tester, with its heater and fans, is relatively inexpensive; however, it has several major limitations.
For example, as the number of transistors within a single IC-chip increases, the maximum amount of electrical power which the IC-chip dissipates also increases. Thus, a point is eventually reached where the maximum variation in power dissipation of multiple IC-chips on several printed circuit boards is simply too large to be regulated by convection with air.
Also, it sometimes is desirable to sequentially test different subsets of the IC-chips which are held on the printed circuit boards; rather than test all of the IC-chips at the same time. But when the number of IC-chips that are being tested changes from a small subset to a large subset, then a large step increase will occur in their total power dissipation. This step increase occurs because the IC-chips that are being sent the test signals dissipate a much larger amount of power than the IC-chips that are not being sent the test signals. Similarly, when the number of IC-chips that are being tested changes from a large subset to a small subset, then a large step decrease in their total power dissipation will occur.
The above step increase and step decrease in power dissipation presents a particularly difficult problem because while the testing occurs, the temperature of the IC-chips needs to be precisely maintained within just a few degrees of a set point temperature. However, when power dissipation of the IC-chips takes a step up, the amount of heat which must be removed from the IC-chips in order to keep their temperature constant increases rapidly. Likewise, when the power dissipation of the IC-chips takes a step down, the amount of heat which must be added to the IC-chips in order to keep their temperature constant decreases rapidly.
Currently in the integrated circuit industry, there is a need for a temperature control system which can maintain the temperature of multiple IC-chips within a few degrees of a set point temperature while their total power dissipation undergoes step increases and step decreases of over twenty-kilowatts. Accordingly, a primary object of the present invention is to provide such a system.
In accordance with the present invention, a system for maintaining the temperature of IC-chips near a set point, while the IC-chips undergo large step increases and large step decreases in power dissipation as they are tested, has the following structure:
1) a hot fluid circuit in which a hot fluid circulates from a reservoir through heat exchangers and back to the reservoir, and in which the heat exchangers transfer heat by conduction between the hot fluid and the IC-chips;
2) an input temperature sensor which generates an input temperature signal that indicates the temperature of the fluid flowing into the heat exchangers;
3) an output temperature sensor which generates an output temperature signal that indicates the temperature of the fluid flowing out of the heat exchangers;
4) an electric heater which adds heat to the fluid returning to the reservoir as a function of both the input temperature signal and the output temperature signal; and,
5) an analog valve which adds a cold fluid to the reservoir as a function of both the input temperature signal and the output temperature signal.
In one mode of operation, the heater adds heat to the fluid that is returning to the reservoir when either one of two conditions occur. The first condition is that the input temperature is less than a selectable set point temperature; and the second condition is that the output temperature is less than the set point temperature. The above addition of heat based on output temperature via the second condition provides a xe2x80x9ctoo cold look aheadxe2x80x9d feature which prevents fluid that gets too cold in the heat exchangers from being put back into the reservoir. If fluid which is too cold is put into the reservoir, then the temperature of all of the fluid in the reservoir will need to be corrected; but that is a slow process because the fluid in the reservoir has a large thermal mass.
In another mode of operation, the analog valve adds cold fluid to the reservoir when either one of two conditions occur. The first condition is that the input temperature exceeds the set point temperature; and the second condition is that the output temperature exceeds the input temperature. The addition of cold fluid based on output temperature via the second condition provides a too hot look ahead feature which prevents fluid that gets too hot in the heat exchangers from being put back into the reservoir by itself. Thus, the slow process of cooling all of the fluid in the reservoir is avoided.
Due to the above xe2x80x9ctoo cold look aheadxe2x80x9d and xe2x80x9ctoo hot look aheadxe2x80x9d features, the temperature of the IC-chips is controlled extremely accurately. In one particular embodiment, the temperature of the fluid which flows into the heat exchangers is held within xc2x11xc2x0 C. of a selectable set point temperature TSP. Further, this small variation in fluid temperature is achieved while the power dissipation of all of the IC-chips steps up and/or steps down by over twenty-kilowatts.